Composition and process for etching magnesium



Patented Sept. 11, 1962 3,053,719 COMPOSITION AND PROCESS FOR ETCHIN GMAGNESIUM Albert R. Jones, Lynwood, and Quentin H. Coffman,

Manhattan Beach, Calif., assignors, by mesne assignments, to PurexCorporation, Ltd., a corporation of California Filed Dec. 8, 1958, Ser.No. 778,850 11 Claims. (Cl. 156-18) This invention relates to thechemical etching or chemical milling of magnesium and its alloys, and ismore particularlyconcerned with the provision of a novel composition andprocess for the chemical etching of magnesium and its alloys, and theetched article produced by such process.

The term magnesium employed throughout the specification and claims isintended to denote magnesium in its pure or commercially available formcontaining impurities, or magnesium alloys in which magnesium i thebasic or dominant metal.

The invention will be described below taken in connection with theaccompanying drawing wherein:

FIGS. 1-3 illustrate the disadvantages of prior art magnesium etchingtechniques;

FIG. 4 illustrates the improved etch obtained employing the etchants andetching procedures of the invention; and

FIG. 5 is a graph particularly illustrating the effect of increasingtemperature on the magnesium etching rate employing the principles ofthe invention as compared to prior art technique,

Magnesium etchant systems now in use have a variety of disadvantages.One of the major difficulties which is inherent in these prior artetchants is the dishing of the etched magnesium part. Dishing is causedwhen the center of the etched portion etches faster than the perimeterareas, which results in more magnesium metal being removed in the centerof the part than toward the periphery. Such dishing is illustrated inFIG. 1 of the accompanying drawing wherein the numeral designates a parthaving a central etched portion 12. It will be n ted that center 14 ofthe etched portion is deeper than the peripheral portion 16, so that atthe center the metal is considerably thinner as indicated at 12a than atthe peripheral portion 1212. Thus, for example, the center portion ofthe etch may have a thickness of only 75 mils, whereas the peripheralportion may have a thickness of as much as 125 mils. The aforementioneddishing problem is caused by a differential rate of attack on themagnesium metal surface, resulting from a temperature gradient in thepart. Dishing therefore is a function of the etch rate. In order toavoid this dishing problem, it is necessary to have a minimum change ofetch rate as a function of temperature.

Another problem encountered in the prior art etching of magnesium isreverse dishing. In this situation, the center area of the etchedportion etches slower than the peripheral areas, thus producing a raisedarea in the center of the etched portion. Thus, for example, viewingFIG. 2 of the drawing, it is noted that the central portion 12 of theetched article 10 has a raised portion 14 as compared to the peripheralportion 16'. In this situation, the thickness of the raised central areaindicated at 12a may be, for example, 125 mils, whereas the thickness atthe peripheral portions indicated at 12b may be only 75 mils. The majordisadvantage resulting from reverse dishing is that stress areas are setup in the fillet portions at 16', producing weakened sections at thesepoints.

Another major problem encountered in the prior art etching of magnesiumis the control of surface smoothness. Magnesium etchants heretofore haveproduced surfaces that, while acceptable for certain purposes, aregenerally of a rough texture not acceptable for certain applications.Thus, magnesium etchants heretofore employed have produced roughsurfaces having a profilometer reading of R.M.S. and above. Theprofilometer is an instrument which is employed to measure roughness ofsurface in micro-inches. The profilometer is described in the articleHow Smooth is Smooth by B. C. Brosheer in American Machinist, September9 and 23, 1948. A standard profilometer is the profilometer Type QAAmplimeter manufactured by the Micrometrical Manufacturing Company, AnnArbor, Michigan. R.M.S. is an abbreviation for micro-inches root meansquare. A rough surface is undesirable because the greater surface areaof metal which is exposed is more susceptible to exidation The hills andvalleys of a rough surface also contribute to low fatigue life of amagnesium part compared to a smooth surface. It is desirable to formsmooth etched surfaces on a magnesium part; said surfaces having asurface smoothness of 40 to 50 R.M.S.

Another major fault in the prior art etching of magnesium is that theseprior art techniques produce an etched part having an etch factor ofless than 1. The term etch factor is the distance or the amount ofundercutting between the edge of the mask and the wall of the etchedportion of the part, divided by the vertical depth of the etch. Thus,for example, viewing FIG. 3 of the drawing where numeral 20 represents apart that has undergone etching and 2.2 is the etched portion thereof,the numeral 24 indicates the extent of lateral etching at the walls ofthe part adjacent the mask 26, and the etch factor is the distance Abetween the edge 27 of the mask and the lateral extent 24- of the etchedwall, and the value B is the depth of the etch. For example, the etchfactor for a magnesium part etched according t present prior artpractice may be on the order of about 0.5. Such an undercut part or apart with an etch factor of substantially less than 1, that is where A lB is substantially less than 1, produces an area of stress Weakness atthe base of the fillet 24'.

It is desirable to obtain an etch on magnesium where the etch factor isapproximately 1. This is accomplished Where the rate of lateral etchingis about the same as the rate of vertical etching, so that the amount ofundercutting at the mask and the depth of etch are approximately equal.The processes and compositions of our invention are capable of producinga controlled etch factor of any desired value which may be higher than0.5 and also higher than 1, the process being preferably controlled toobtain an etch factor of about 1.

Another difficulty of prior art etchant systems is that in such systemsthere is produced a high etch rate, for ex ample, at temperatures ofabout F. the etch ratio of prior art etchants is many times greater thanthe etchants of our invention, and the etch rates of such prior artetchants increases rapidly as the temperature increases, particularly attemperatures above 110 F. This high etch rate increase contributes tosevere dishing and to production of a rough surface, both undesirablefeatures as noted above. For example, in a magnesium etchanc composedessentially of sulfuric acid, an etch rate in excess of 7 mils perminute is obtained with the resultant panel havinga rough surface,uncontrolled etch line boundaries, and severe dishing. The best priorart etchants known to us will, under like conditions, give etch rates of3 to 4 mils per minute in the region of about 110 F. Further, as theetching temperature increases, etch rate increases to an uncontrollablevalue. In order to produce smooth surfaces without dishing, it has beenfound necessary to maintain an etch rate (the number of mils (.001 inch)of magnesium metal removed over a period of one minute), of about 2 milsper minute or less, and preferably on the order of about 1 mil perminute, and it is highly desirable to maintain such etch rate over avarying temperature range as etching proceeds.

It is accordingly an object of the present invention to provide acomposition and process for production of an etched magnesium part whichhas substantially no dishing and which is uniform over the entiresurface.

A further object is to provide a composition and process for etchingmagnesium to produce a surface smoothness over the entire etched areawhich is unobtainable by prior art techniques.

Still another object is to afford a composition and process for etchingmagnesium in which the etch rate across the whole surface, that is, thedepth of etch across the whole surface, is maintained uniform.

Yet another object is the provision of a composition and process foretching magnesium wherein the lateral etch rate is substantially equalto the vertical etch rate.

It is another object of the invention to provide a composition andprocess for etching magnesium, so as to produce an etched surface havingan etch factor as close as possible to 1.

It is still another object to afford a composition and process foretching magnesium wherein the rate of etching at high temperature is notincreased by increases in temperature, and wherein the etch rate issufficiently slow and controllable even at high temperatures so as toobtain smooth, uniform surfaces.

It is a still further object to provide a process for rejuvenatingetching baths of the type particularly employed in the invention asthese baths become loaded with'magnesium sulfate during the process ofetching, so as to maintain proper acid concentration, magnesium sulfateconcentration and rate of etch to obtain the aforementioned advantages.

Yet another object is the production of an article of manufacture in theform of a chemically etched magnesium product in which the depth of etchacross the en'- tire etched surface is uniform, wherein the etch factoris approximately 1, and which is neither dished nor reverse dished, andwhich has a smooth surface over the entire etched portion.

Other objects and advantages will be apparent from the followingdescription of the invention.

We have found that by etching a magnesium part with an aqueous sulfuricacid solution of a sulfate, preferably sodium sulfate or sodiumbisulfate, and containing sodium xylene sulfonate, and preferably,although not necessarily, includinga small amount of an hydroxycarboxylic acid such as -citric acid, the etch rate is materiallyreduced to a value such that the etched surface of the part is smoothover the entire-bottom area of the etched portion and also along thesidewalls. When it is particularly desirable to fotm 'smooth etchedsurfaces on a magnesium part, the process and compositions of ourinvention are capable of producing a smoothness of 40 to 50 R.M.S. andas low as 20 to 25 R.M.S., or any variations thereof as may be desired.Also, "by practice of the invention the depth of etch can be controlledso that it is substantially the same at the center and periphery of theetched surface due to the uniform etch rate. Thus, regardless of theshape of the'original'exterior surface, e.g. whether curved, fiat orirregular, the bottom of the etched depression formed in such exteriorsurface will have a depth from said surface which depth is substantiallyuniform at all portions of the depression bounded by the peripheralsides thereof. Further,-by practice of the invention, dishing or reversedishing of the etch can bev prevented, both of which occur in the priorart processes. Moreover, the uniform etch rate in both the lateral andvertical directions obtained by the invention can result in a filletbordering the etched portion, said fillet having an etch factor ofabout 1. 7

Of particular significance, it has been found that by employing theetchant composition and process of the invention at temperatures ofetching of about 110 F. and above, as the temperature is raised the rateof etching actually is reduc d. This is a surprising phenomenon inasmuchas conventional magnesium etchants produce excessive etch rates at theseelevated temperatures, and these etch rates increase with increase intemperature. The reduced rate of etching according to the inventionprocess at these high temperatures results in controlled etching andaids production of uniform, smooth surfaces With an etch factorapproximately 1, and the minimum fluctuation of etch rate withtemperature according to the invention also aids in preventing theaforementioned dishing.

It has been found necessary for proper functioning of the inventioncomposition to employ a substantial amount of either sodium sulfate orsodium bisulfate in the highly acid aqueous sulfuric acid solution.Since sodium sulfate in sulfuric acid is known to be converted to sodiumbisulfate, this latter material can be initially employed in place ofthe sodium sulfate. In any event, it has been found essential that asubstanital proportion of the sulfate ion be present furnished by thesulfuric acid and the sodium sulfate or sodium bisulfate, in conjunctionwith the xylene sodium sulfonate. In this respect it has been found thatalkali metal sulfates or bisulfates other than the sodium salt are notsuitable. Thus, for example, where potassium sulfate is employed inplace of sodium sulfate, a rough, unsatisfactory surface is obtained.

The sodium xylene sulfonate of the invention composition aids inproducing a desirably slow etch rate on the order of about 1 mil perminute, which, in turn, is of major importance in obtaining the smoothlyetched magnesium surfaces of the invention. Further, it has been foundthat apparently the sodium xylene sulfonate is responsible for thedecreasing etch rate as a function of increasing temperature especiallyat etchant temperatures of the order of F. and above.

Although not a necessary ingredient of the invention composition foretching magnesium, the inclusion of a simple hydroxy carboxylic acidimproves the etching-a0- tion of the composition. Thus, for example, theaddition of a small amount of citric acid appears to improve filletformation and also to improve surface smoothness. In place of citricacid, other hydroxy carboxylic acids such as tartaric, malic, orglycollic acid may be employed. The polyhydroxy carboxylic acids arepreferred. It was observed that as the etching of magnesium partsproceeds in the aforementioned etchant composition, the etched surfacesmooths considerably. We found that the increasing content of magnesiumsulfate produced in the bath during the etching period contributed tothis improved functioning of the bath. We have accordingly observed thatwhen magnesium sulfate is added to the initial composition, theforegoing result can be accomplished more quickly to produce panelshaving good fillets, excellent linedefinition, no dishin' and a surface,for example, of 40-40 R.M.S., meaning a smoothness of 40 R.M.S. with thegrain of the metal and a smoothness of 40 R.M.S.'across the grain of themetal. However, it will be understood that since, as pointed out above,magnesium sulfate is formed in the 'bath as etching proceeds, it is notnecessary that the magnesium sulfate be present in the initial bath,although the initial presence of this material is preferred. Thus, theinitial bath need not contain magesium sulfate, but in preferredpractice magnesium sulfate may be included in the original bath, e.g. byadding this material directly thereto, or by adding a portion of asolution which was previously used in etching magnesium and thuscontains magnesium sulfate.

In order to obtain the improved results of the invention, it has beenfound that the above ingredients should be present in the solution oretching bath preferably in Citric acid or equivalent acid to 1 Sodiumxylene sulfonate 1.5 to 6 Sulfuric acid66 Baum 16to 23 Water, remainder.

However, for obtaining best results, the following preferred proportionsare employed:

TABLE 11 Preferred range, Material percent by weight MgSO .7H O 4 to 8NaSO, or NaHSO (anhydrous basis)- 12 to 16 Citric acid or equivalentacid 0.4 to 0.6 Sodium xylene sulfonate 3 to Sulfuric acid66 Baum 18.4to 22.4 Water, remainder.

As aforementioned, the magnesium sulfate can be present initially in thesolution, but if not, it will form as etching proceeds. The presence ofcitric acid or equivalent acid noted above is preferred, but is notnecessary.

If proportions of ingredients greater or less than those set forth inTable I are employed, the results of the invention including uniform,low etch rate, smoothly etched surfaces and walls, and good fillets, aregenerally not obtainable. When employing the proportions within thepreferred ranges of Table II, generally the best results are produced asregards smoothly etched surfaces having a 40 to 50 R.M.S. or lessproiilometer reading with and across the grain, uniform depth of etchover the etched surface, good fillet formation, and etch factor of about1.

The various components of the solution can be added separately to waterto form the working solution, or those components which are generallyobtainable in dry form can be mixed and packaged, and the resultingcomposition added to an aqueous sulfuric acid solution to produce theinvention composition. Thus, for example, sodium sulfate and xylenesodium sulfonate, with or without citric or equivalent acid, and with orwithout magnesium sulfate, can be mixed in certain proportions, and theresulting mixture added to aqueous sulfuric acid to produce theaforementioned working etching solutions. Such mixtures excludingmagnesium sulfate can also be used as make-up for a spent solution, asdescribed hereinafter.

For example, an overall and preferred range of proportions of such solidcompositions are as follows:

Compositions within the ranges specified in Table III can be added tothe proper proportions of sulfuric acid and water to form workingsolutions substantially as given above in Tables I and II. It will beunderstood that the relative proportions of each of the ingredients ofthe corn-positions in the dry form and the concentration of 6 the drymixture employed in the solution should be chosen so that when the drymixture is added to sulfuric acid and water, the percentage of therespective ingredients in solution will be preferably within the rangesspeci fied in Tables I and II.

The operational temperature of the etchant baths formed from the etchantcompositions of the invention is preferably in the range of about to F.,most desirably about 110 F. However, as pointed out above, as thetemperature of the etchant is increased above l10 F. during the etchingoperation, the etch rate will decrease rather than increase and, hence,no danger of an inordinate rate of etching is present, with theattendant disadvantages noted above, including formation of a roughetched surface.

This phenomenon is brought out in Example 1 below, illustrating theprinciples of the invention, and described in connection with FIG. 5 ofthe drawings, showing a graph with etch rate in mils per minute againsttemperature.

Example I The following compositions A, B, C andD were prepared andAZ-31B magnesium alloy parts of the same size and shape wererespectively etched in each of solu- It will be noted from FIG. 5 thateach of the invention compositions containing sodium xylene sulfonateproduced a curve showing an etch rate less than 1%. mils per minute atetching temperature of 110 F., and a decrease in etch rate as etch bathtemperature increased from 110 to 130 F., whereas the composition D,similar to compositions A, B, and C, but not containing sodium xylenesulfonate, commenced etching at a high etch rate of almost 3 mils perminute and the etch rate increased relatively rapidly with increase intemperature to an etch rate of 4 mils per minute at 130 F.

It will be seen also that as the amount of sodium xylene sulfonate isincreased from 2% in composition B to 3% in composition A, and 4% incomposition C, the corresponding curve is lowered to an etch rateapproaching the most desirable rate of about 1 mil per minute. See thecurve for composition C containing the largest amount (4%) of sodiumxylene sulfonate. That is, while at 110 to F. the etch rate forcompositions A to C is above 1 mil per minute, between 120 F. and F. theetch rate either closely approaches or drops slightly below 1 mil perminute. Etch rates slightly below 1 mil per minute, e.g. about 0.8 milper minute, are suitable for producing the invention results. The use oflarger amounts of sodium xylene sulfonate than that employed incomposition C is impractical from the economic standpoint, since thecost of such larger amounts of sodium xylene sulfonate in the bathbecomes prohibitive, and the results obtained are not significantlybetter than those obtained using composition C.

Further, it will be observed from FIG. 5 that as the amount of sodiumxylene sulfonate is increased the effect of increase of temperature from110 to 130 F. is lessened; that is, the curve tends to flatten out sothat only a relatively small decrease in etch rate occurs as temperatureincreases from 110 to 130 F., and the etch rate remains close to 1during this increase in temperature. Compare, for example, therelatively flat curve for composition -C as compared to the curve forcomposition A or B, containing much smaller amounts of sodium xylenesulfonate.

The etched magnesium surfaces produced by compositions A, B and C weresmooth with profilometer readings varying from about 40 to about 50R.M.S. both with and across the grain, whereas the etched surfaceproduced in etchant D not containing sodium xylene sulfonate had a roughsurface of about 80 R.M.S. both with and across the grain. Further, theetched surfaces employing compositions A to C were not dished (see FIG.

. 4), whereas etching with composition D produced a dished surface. SeeFIGS. 1 and 2.

Also, it is noted that smoother surfaces were obtained using compositionC than compositions B and A. Thus the flatter the curve and the closerit is to the 1 mil per minute etch rate, the most desirable are theresults produced.

However, where, for example, the non-dishing advantage of the inventioncan be sacrificed, and it is desired that the etched magnesium surfacebe smooth regardless of dishing, the amount of sodium xylene sulfonateemployed can be reduced below the 2% by weight of solution ofcomposition B, down to as low as about 1.5%. See Table I. This willresult in an etch rate above 1.5 when operating at temperatures between110 and 130 F., the etch rate increasing with decrease in sodium xylenesulfonate content. While this increased etch rate may produce somedishing, the etched surface is smooth and uniform as contrasted to therough etched surface produced in the total absence of sodium xylenesulfonate in the etchant, e.g. when employing a composition such as thatof composition D.

This composition was added to Water together with magnesium sulfate and66 Baum sulfuric acid solution, employing per gallon of solution 2 lbs.of the composition of Table IV, 0.70 lb. MgSO .7H O and 2.35 lbs.66.B'aum sulfuric acid, to form 100 gallons of the workingzsolutionhaving the approximate composition shown in Table V below.

TABLE V Percent by weight of solution MgSO .7H O 6.2 Na SO 13.7 Citricacid 0.4 Sodium xylene sulfonate 4.0 Sulfuric acid-66 Baum 20.9 Water54.8

AZ-31B magnesium alloy panels with dimensions of 9" x 19" were etched toa depth of 215 mils in the etchingsolution of Table V, at operatingtemperature'of 1.10 to 130 F. in aperiod of 185 minutes.

The etched panels :had a profilometer reading of 40-40, indicating anexcellent smooth surface, excellent line definition and well shapedfillets. The etched panels hada uniform depth of etch across the wholeetched surface which was neither dished nor reverse dished, and thepanels had an etch factor on the order of about 1.05. The etch producedis illustrated in FIG. 4, wherein the panel has a gently rounded wall 31substantially in the form of a quarter circle with the undercut Asubstantially equal to the depth of etch B. The bottom 32 of theetchedsurface 'is smooth and uniform, and the depth of the etch from'thesurface 38 to the bottom 32 of the etched depression is substantiallyuniform at all portions of the bottom 32,at all angles to the sectionillustrated in FIG;

4, the metal having the same thickness at the center 34of the etch andat the peripheral edge 36 thereof.

Similar etching runs were made with the following magnesium alloys:HK-31A, HM-Zl and EX61 (sand cast). Results similar to those obtainedusing AZ31B a-lloy were obtained.

Example 3 The procedure of Example 2 is carried out except that citricacid is omitted from the composition and the amount of water isincreased to 55.2%. The results obtainable are similar ,to those ofExample 2 except that the fillets are not quite as well formed as in theetched panels of Example 2.

' Example 4 Example 5 When tartaric acid or malic acid is substitutedfor the .Citric acid in the composition of Example 2 substantially thesame results as in Example .2 are obtainable.

In order that the invention etchants function in the most eflicientmanner, it is necessary that a maskant be employed which has goodadhesion to the metal surface. Otherwise, release of the mask from themetal surface results, causing variable etch factor along the fillet,i.e. at the etch line along the edge of the mask, to produce anon-linear, e.g. curved .or bowed line, which is undesirable. Thus, forexample, the etching composition of the invention can operateefiiciently using the improved maskant composition of the co-pendingapplication Serial No. 579,717, filed April 23, 1956, of Atkins andEdds, comprising chloroprene, polymer, carbon black and a phenolic, e.g.phenol-aldehyde, resin.

As previously noted, during the period of etching, the etchant ionizesmagnesium into solution, and magnesium sulfate is formed by the etchingreaction. As the etching proceeds, the solution becomes loaded withmagnesium sulfate to a point where the etchant is reduced ineffectiveness. At this point the etch rate diminishes and the surfacebecomes rough. In the operation .of the invention employing thecompositions aforementioned, we have found that by reducing theconcentration of the magnesium sulfate, preferably .by removal of aportion of the bath, and adding additional composition of the inventionnot containing magnesium sulfate, the etching bath may thus berejuvenated and continued to be employed as effectively as at itsinitialstages of use. Hence, by prac-' ticing the rejuvenationprocedure, an additional feature of the invention process, the etcha'ntcomposition of the invention does not reach an overloaded, inoperablecondition.

Also, as etching proceeds, and as magnesium goes into solution, thesulfuric acid is consumed in forming magnesium sulfate. We have foundthat when the acid number (milliequivalents of sodium hydroxidenecessary to,

neutralize 5.0 cc. of etchant to bromothymol blue endpoint) employed asa measure of acid concentration, is depleted by the chemical milling oretching of the magnesium parts, then depending on the .acid number ofthe resulting bath, .a certain proportion of the bath is discarded .anda certain amount of etchant, excluding magnesiumsulfate, e.g.,the-composition noted above in Table IV, is added to the remaining bath,together with a certain amount of sulfuric acid, as will be noted in the.rejuvenation chart in Table VI below.

Where citric acid is not employed in the composition 9 of Table IV thepercentage of sodium sulfate is increase to 77.9%.

- Example 6 In Table VI below is set forth in chart form the acidnumber, the percentage of the original bath to be disposed ofcorresponding to said acid number, the amount of etchant composition ofTable IV which is to be added corresponding to such acid number, and thepounds of 66 Baum sulfuric acid to be added per 100 gallons of bath.

TABLE VI Pounds of Eteh- Pounds of 66 Percent of ant from Table Be His04 to Acid N 0. Bath to be IV to be added add per 100 Dumped per 100Gallons Gallons of Bath of Bath Example 7 The 100 gallons of etch bathof Example 1, were continued to be used for etching additional magnesiumparts until the acid number of the bath decreased to 26.0. Etching wasinterrupted and following the chart of Table VI, about 17% by weight ofthe total bath was discarded, and to the remaining bath was added 38.6pounds of the etchant composition of Table IV and about 47 pounds of 66Baum sulfuric acid. The components of the resulting bath were now withinthe proportions given in Tables I and II above.

The composition and process of the invention possess, in addition toother aforementioned advantages, the following advantages overconventional etchants and processes for etching magnesium: The inventioncomposition preferably operates in a temperature range of 105-115" F.,and tap water at ambient temperatures passed through cooling coils inthe etching tank is sufficient for cooling. Between 105 and 115 F. etchrate diminishes, and no dishing at these elevated temperatures occurseven over large surfaces. Conventional magnesium etchants on the otherhand, are operated at a lower temperature of 85 F. to 100 F., and thislower operating range requires refrigeration for cooling. At the latteroperating temperatures, dishing of the etched part takes place, and attemperatures above 100 F. employing conventional etchants, the etchantproduces increased dishing, uncontrolled etch rate and poor surface.With the etching composition of the invention smooth surfaces withprofilometer readings of 40 to 50 are obtainable, whereas withconventional magnesium etchants, rough etched surfaces are obtainedgenerally with profilometer readings in excess of 100. When employingthe bath rejuvenation procedure of the invention, the magnesium sulfateconcentration does not drastically change and the bath can be operatedalmost indefinitely. In conventional magnesium etching procedures thegradually increasing magnesium sulfate concentration builds up to suchan extent that the etchant soon becomes uncontrollable.

While in preferred practice it is desirable to operate according to theinvention in a manner to attain all of the aforementioned inventionadvantages, including smooth uniform surface, absence of dishing,controllable etch rate, etch factor on the order of about 1, good linedefinition, and well formed fillets, it will be understood that undersome conditions of practice of the invention only one or more, but lessthan all, of the aforementioned advantages may be realized, particularlywhere it is not necessary that all of the advantages be attained inetching a particular magnesium part. Thus, for example, where it isdesirable to produce a smoothly etched part regardless of dishing, thatis, where absence of dishing is not important, the invention processemploying the invention etchant can be carried out in such a manner, aspreviously described, that a smooth surface is attained even though thesurface may be dished. Likewise, for example, where a uniform undishedsurface is of major importance and smoothness of surface is relativelyimmaterial, the principles of the invention may be applied to attain anundished surface and uniform depth of etch, the smoothness of thesurface corresponding to a profilometer reading in excess of say 50.

It will be understood that for different magnesium alloys etched by agiven composition and under a given set of operating conditionsaccording to the invention, the results as to smoothness, non-dishingand other advantages obtained by practice of the invention, as notedabove, will vary to some extent.

The compositions of representative illustrative magnesium alloys whichcan be etched according to the invention are as follows, the quantitieslisted being in per cent, the balance of the composition in each casebeing magnesium: magnesium alloy AZ-31B containing 2.5-3.5 aluminum, .2manganese, .7l.3 zinc, .04 calcium, .3 silicon and .05 copper; the alloyHK-31A containing 2.5-4 thorium and .51.0 zirconium; the alloy HM-21containing 1.5-2.5 thorium and .35-.8 manganese; and the alloy EK-4lcontaining .15 manganese, .30 zinc, .4-1 zirconium and 3.0-5.0 rareearths.

The term consisting essentially of as used in the definition of theingredients present in the compositions claimed is intended to excludethe presence of other materials in such amounts as to interferesubstantially with the properties and characteristics possessed by thecomposition set forth but to permit the presence of other materials insuch amounts as not substantially to affect said properties andcharacteristics adversely.

While we have described particular embodiments of our invention for thepurpose of illustration, it should be understood that variousmodifications and adaptations thereof may be made within the spirit ofthe invention as set forth in the appended claims.

We claim:

1. A composition of matter which when added to aqueous sulfuric acidforms an etching solution for magnesium, which consists essentially ofabout 45 to about 84 parts by weight of a member of the group consistingof sodium sulfate and sodium bisulfate, about 10 to about 25 parts byweight of sodium xylene sulfonate, and a hydroxy carboxylic acid of thegroup consisting of citric, tartaric, malic and glycollic acids, in aneffective amount up to about 4 parts by weight of said composition.

2. A composition of matter which when added to aqueous sulfuric acidforms an etching solution for magnesium, which consists essentially ofabout 71 to about 81 parts by weight of sodium sulfate, about 16 toabout 24 parts by weight of sodium xylene sulfonate, and about 2 toabout 3 parts by weight of a hydroxy carboxylic acid of the groupconsisting of citric, tartaric, malic and glycollic acids.

3. A composition of matter which when added to aqueous sulfuric acidforms an etching solution for magnesium, which consists essentially ofabout 75.7% by weight of sodium sulfate, about 2.2% by weight of citricacid and about 22.1% by weight of sodium xylene sulfonate.

4. An aqueous acid etching solution for magnesium, which consistsessentially of about to about 20 parts by weight of a member of thegroup consisting of sodium sulfate and sodium bisulfate, about 1.5 toabout 6 parts by weight of sodium Xylene sulfonate, and about 16 toabout 23 parts by weight of sulfuric acid calculated as 66 Baum sulfuricacid, based on 100 parts by weight of the solution.

5. A process for etching a magnesium part at an etch rate of about 0.8to about 1.5 mils per minute to obtain a smooth etched surface, whichcomprises treating said part in an aqueous acid solution as defined inclaim 4. 6. An aqueous acid etching solution as defined in claim 4,wherein said member is sodium sulfate, and including magnesium sulfatein an effective amount'up to 12 parts by weight, calculated as MgS O -7HO, and citric acid in an effective amount up to about l'part by weight,based on 100 parts by weight of the solution.

7. A process for etching a magnesium part at an etch rate of about 0.8to about 1.5 mils per minute to obtain a smooth etched surface having aprofilometer reading of about 40 to about 50 R.M.S. with and across thegrain of the metal and an etch factor of about one, which comprisestreating said part in an aqueous acid solution as defined in claim 6,the temperature of saidsolution ranging from about 110 F. to about .130F.

8. A composition of matter which when added to aqueous sulfuric acidforms an etching solution for magne- 12 sium, which consists essentiallyof about 45 to about 84 parts by weight of a member of the groupconsisting of sodium sulfate and sodium bisulfate, and about 10 it,

about 25 parts by weight of sodium xylene sulfonate.

9. A composition of matter as defined in claim 8, wherein said member issodium sulfate.

10. A composition of matter as defined in claim 1, wherein said hydroxycarboxylic acid iscitric acid.

11. A composition of matter which when added to aqueous sulfuric acidforms an etching solution for magnesium, which consists essentially ofabout 45 to about 84 parts by weight of a member of the group consistingof sodium sulfate and sodium bisulfate, about 10 to about 25 parts byweight of sodium xylene sulfonate, a hydroxy .carboxylic acid of thegroup consisting of citric, tartaric, malic and glycollic acids, in aneffective amount up to about 4 parts by weight of said composition, andmagnesium sulfate in an effective amount up to about parts, calculatedas MgSO -7H O, by weight of said composition.

References Cited in the file of thispatent UNITED STATES PATENTS1,918,545 Hoy July 18, 1933 7 2,018,388 'Ilosterud Oct. 22, 19352,287,050 Miller June 23, 1942 4 2,318,559 Percival May 4, 19432,674,523 McDonald Apr. 6, 1954 2,739,047 Sanz Mar. 20, 1956 2,760,891Nichols Aug. 28, 1956

1. A COMPOSITION OF MATTER WHICH WHEN ADDED TO AQUEOUS SULFURIC ACIDFORMS AN ETCHING SOLUTION FOR MAGNESIUM, WHICH CONSISTS ESSENTIALLY OFABOUT 45 TO ABOUT 84 PARTS BY WEIGHT OF A MEMBER OF THE GROUP CONSISTINGOF SODIUM SUFLATE AND SODIUM BISULFATE, ABOUT 10 TO ABOUT 25 PARTS BYWEIGHT OF SODIUM XYLENE SULFONATE